1. Field of the Invention
The present invention relates to concrete formwork, and more particularly, to prefabricated formwork for the in situ formation of complex curved structures.
2. Description of the Prior Art
It is common practice to build prefabricated wood formwork for complex curved structures, such as draft tubes utilized in hydroelectric power houses. A draft tube conducts the used water immediately downstream of a turbine which allows the water exhausting from the turbine to be diffused.
The draft tube is normally defined in the concrete power house foundations under each turbine. The draft tube defines a circular opening at the base or outlet of the turbine which gradually changes to an ellipse and then to a wide flattened mouth of increased sectional area while the longitudinal axis of the tube changes 90.degree. or less from the vertical to a somewhat horizontal plane. Thus, in order to pour the concrete foundation while defining the draft tube, a giant plug in the form of the tube must be provided in order to allow the concrete to be poured around the plug. As can be seen, the draft tube cannot be precast and transported to the site but must be formed in situ.
According to the handbook "Formwork for Concrete", M. K. Hurd, Fifth Edition, published in 1989 by Committee 347 of the American Concrete Institute, the "plug" or formwork would be made from wood. Also, as noted in this reference, the wooden "plug" can be prebuilt in segments and assembled on site. This allows for easier manufacture of the plug and for easier stripping, and the forms can be reused as a formwork plug on other sites.
However, each wood form is a complicated structure requiring highly skilled artisans. Each form has a complexly curved, three-dimensional contact or mold surface. The mold surface of the form must be smooth in order to provide a fair-faced surface which is free of any anomalies which might cause turbulence in the water flow resulting in cavitation in the tube surface.
The fabrication of such wood forms is also time consuming, requiring anywhere from six to eight weeks for each one to be made. In order to reuse the forms, considerable repair must be made to the wood form after it has been stripped. In view of the inherent flexibility of the wood form, it must be carefully and heavily shored to ensure that its predetermined shape is faithfully maintained.